1. Field of the Invention
This invention relates to a half-tone display system in which, in displaying video signals such as television signals, half-tone displaying of n gradations is achieved per field, the n being positive integer. More particularly, the present invention relates to a half-tone display system for a flat matrix type cathode-ray tube (CRT) in which pictures displayed on the CRT are prevented from flickering such as beat interference.
2. Description of the Prior Art
An example of a conventional flat matrix type CRT is so designed that grid electrodes consist of a plurality of scanning electrodes and a plurality of data electrodes which are arranged to form a matrix structure together with the scanning electrodes. Pulse voltages of predetermined gradations are applied selectively to one of the electrodes in the two different electrode groups so that picture elements designated by the selected scanning electrodes and data electrodes to which the pulse voltage are applied, are caused to emit light.
In this operation, as is well known in the art, electrons emitted from a direct-heating type linear filament are accelerated by the voltages applied to the grid electrodes consisting of the scanning electrodes and the data electrodes, to impinge an anode at a high voltage, whereby a light emitting material such as a fluorescent layer which is formed on the anode by coating, is caused to emit light by the energy of collision.
FIG. 1 shows the grid electrode structure of such a flat matrix type CRT. In the flat matrix type CRT of FIG. 1, the grid electrode structure is a 4.times.4 matrix structure consisting of scanning electrodes X1 through X4 and data electrodes Y1 through Y4. In correspondence to the 4.times.4 matrix structure, fluorescent substances of red, green and blue are applied to the predetermined parts of the anode surface (not shown) as required, so that sixteen (16) picture elements in one field can appear red (R), green(G), and blue (B).
A desired one of the picture elements can be selected by applying voltages to the scanning electrode and the data electrode which are provided correspondingly for the picture element. And only the picture element thus selected can emit light.
FIGS. 2(a) through 2(h) are diagrams for a description of a conventional half-tone display system for the above-described flat matrix type CRT, in which one field is divided into four parts, and, in a 1/4 duty, display is carried out with sixty-four (64) gradations. In FIGS. 2(b) through 2(e), reference characters GX.sub.1 through GX.sub.4 designate the voltage waveforms of scanning signals applied to the scanning electrodes X1 through X4 shown in FIG. 1, respectively. In FIG. 2(f), reference character GY designates one of the voltage waveforms of data signals applied to the data electrodes Y1 through Y4 shown in FIG. 1. As shown enlarged in FIGS. 2(g) and 2(h), while one of the scanning signals, GX.sub.1 for instance, is selected, a signal of sixty-four gradations 0, 1, 2, . . . A, . . . 38, 39, 3A, . . . 3F (=N) in hexadecimal notation is provided as the data signal. In FIGS. 2(f) and 2(h), reference characters TB1 through TB4 designate blanking periods.
In the case where a DC drive source is employed for the above-described CRT, a potential difference occurs between both ends of the direct-heating type linear filament (cathode) resulting in varying the brightness of a picture displayed thereon. In order to eliminate this difficulty, an AC power source is employed. As shown in FIG. 2(a), an AC drive voltage E.sub.f is supplied to the filament to maintain the brightness of the display picture uniform.
Further, in the case where, in displaying ordinary information, half-tone displaying is not required, the scanning electrodes can be scanned at a high speed, and the frequency can be selected as desired. On the other hand, in the case where the half-tone displaying is required to display video signals such as television signals, the picture is such that the scanning is repeated sixty (60) times per second and one (1) field is provided per 1/60 second. Therefore, the repetitive period of the picture coincides with the oscillation period of the drive source, as a result of which noises such as beats in the displayed picture occur.